Self-Supported Porous NiSe2 Nanowrinkles as Efficient Bifunctional

Dec 11, 2017 - Developing low cost, highly active, and stable bifunctional electrocatalysts for overall water splitting is significant for sustainable...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 2231−2239

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Self-Supported Porous NiSe2 Nanowrinkles as Efficient Bifunctional Electrocatalysts for Overall Water Splitting Jie Zhang,† Ying Wang,† Chi Zhang,‡ Hui Gao,† Lanfen Lv,† Lulu Han,† and Zhonghua Zhang*,†,‡ †

Key Laboratory for Liquid−Solid Structural Evolution and Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jingshi Road 17923, Jinan 250061, P. R. China ‡ School of Applied Physics and Materials, Wuyi University, 22 Dongcheng Village, Jiangmen 529020, P. R. China

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ABSTRACT: Developing low cost, highly active, and stable bifunctional electrocatalysts for overall water splitting is significant for sustainable energy systems. Herein, we report the synthesis of three-dimensional porous nickel diselenide nanowrinkles anchored on nickel foam through fabricating nickel oxalate nanosheets on nickel foam by immersion, followed by selenization under a selenium vapor atmosphere. The hybrid material exhibits superior hydrogen evolution reaction and oxygen evolution reaction performances with high activity (low overpotential), favorable kinetics, and outstanding durability in alkaline solutions. An overpotential of merely 166 mV is needed to reach 10 mA cm−2 for hydrogen evolution reaction and 235 mV for oxygen evolution reaction. The NiOOH species formed at the NiSe2 surface serves as the catalytic sites. Moreover, the electrolyzer only needs a cell voltage of 1.64 V to deliver 10 mA cm−2 for overall water splitting and shows excellent long-term stability (80 h at 10 mA cm−2). The unique porous nanowrinkle structure, improved electrical conductivity, fast charge transfer kinetics, and large electrochemical surface area are responsible for the highly active and stable electrocatalytic performance toward hydrogen evolution reaction/oxygen evolution reaction. KEYWORDS: Porous NiSe2 nanowrinkle, Bifunctional electrocatalyst, Hydrogen evolution reaction, Oxygen evolution reaction, Overall water splitting

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INTRODUCTION With the exhaustion of fossil fuels and serious environmental problems, it is urgent to search CO2-free and sustainable energy sources.1,2 Electrocatalytic water splitting has been regarded as a secure and sustainable approach to generate clean hydrogenfuel energy,3−5 and it is divided into two half reactions: hydrogen evolution reaction (HER) at the cathode and oxygen evolution reaction (OER) at the anode. The theoretical value of overall water splitting is 1.23 V, but larger overpotential is needed because of the thermodynamic uphill of both reactions.6 Thus, it is critical to develop efficient electrocatalysts to reduce energy consumption. Currently, the state-of-the-art catalysts for HER and OER are Pt-based and Ir/Ru-based materials, respectively. However, limited reserves and high cost hinder their large-scale applications.7,8 Consequently, inexpensive and durable non-noble electrocatalysts have received much attention, such as phosphides,9−14 sulfides,15−17 selenides,18−20 © 2017 American Chemical Society

carbides,21,22 and nitrides22−24 for HER and hydroxides,25−27 oxides,28−31 and chalcogenides32,33 for OER. HER catalysts usually work under acidic conditions while they require higher overpotentials in alkaline solutions.34−36 In contrast, catalysts for OER perform well in alkaline solutions. Thus, it is difficult to integrate both HER and OER catalysts into a full cell which exhibits superior overall performance. Since only a few OER catalysts are insoluble in acid solutions, developing novel and efficient bifunctional catalysts for overall water splitting in alkaline solutions is of great significance.37 For example, NiFe layered double hydroxide (LDH) as bifunctional electrocatalysts can deliver 10 mA cm−2 at a cell voltage of 1.7 V in an alkaline medium.3 Although tremendous efforts have Received: October 10, 2017 Revised: November 25, 2017 Published: December 11, 2017 2231

DOI: 10.1021/acssuschemeng.7b03657 ACS Sustainable Chem. Eng. 2018, 6, 2231−2239

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ACS Sustainable Chemistry & Engineering

the tube furnace,43 with ∼0.5 g of selenium powder placed at the upstream side of the furnace as the Se source. Impurity gas in the furnace was purged with Ar gas (purity, 99.999%) for 30 min. Afterward, the furnace was programmed to the desired temperature (350 °C) at a heating rate of 10 °C min−1 and kept at this temperature for 2 h. After the growth, the furnace was automatically turned off. Finally, the furnace naturally cooled down to room temperature. The Ar gas was kept flowing at a rate of 80 sccm to transport selenium to the substrate during the whole heat treatment process. The treated Ni foam was then washed with deionized water and dried in air. In addition, a series of sintering parameters (300 °C 2 h, 350 °C 1 h, 400 °C 1 h, and 400 °C 2 h) were conducted to synthesize NiSe2/Ni for comparison. Characterizations. X-ray diffraction (XRD) measurements were performed on an XD-3 diffractometer (Beijing Purkinje General Instrument Co., Ltd., China) equipped with Cu Ka radiation to analyze the phase constitution. The microstructure and morphology of the as-prepared samples were characterized by scanning electron microscopy (SEM, FEI QUANTA FEG 250) and transmission electron microscopy (TEM, FEI Tecnai G2). The selected area electron diffraction (SAED) patterns were also obtained to characterize the crystalline nature of the synthesized samples. Surface elemental information was detected by X-ray photoelectron spectroscopy (XPS, ESCALAB 250). Electrochemical Measurements. The electrochemical performance of HER and OER was investigated in a three-electrode system through a potentiostat (ZAHNER, Zennium). The linear sweep voltammetry (LSV) was conducted at a scan rate of 5 mV s−1 in 1.0 M KOH using a graphite rod and Ag/AgCl (4 M KCl) as the counter and the reference electrode, respectively. The 3D porous NiSe2/Ni was used as the working electrode directly. Before HER measurements, high-purity N2 gas was used to purge the system for at least 30 min to ensure the saturation of N2 in the electrolyte. During the test, the system was continuously purged with N2. For comparison, the electrocatalytic performance of the NiC2O4/Ni, a bare Ni foam, and commercial Pt/C (40 wt % Pt, Johnson’s Matthey) loaded onto a glassy carbon electrode (GCE) (mass loading: 0.2 mg cm−2) was also measured. Before OER measurements, O2 was purged into the 1.0 M KOH for 0.5 h to saturate the electrolyte and maintained during the OER measurement. The electrodes were electrochemically activated with repetitive cyclic voltammetry (CV) for 20 cycles at 50 mV s−1 from 1 to 1.6 V vs reversible hydrogen electrode (RHE) in 1.0 M KOH. The NiC2O4/Ni, a bare Ni foam, and commercial IrO2 loaded onto a GCE (mass loading: 0.2 mg cm−2) were employed for comparison. The overall water splitting was performed in a twoelectrode system in 1.0 M KOH. The long time durability test was performed at 10 mA cm−2 for HER, OER, and overall water splitting. Electrochemical impedance spectroscopy (EIS) measurements were recorded under an excitation voltage of 5 mV within the frequency range from 10−2 to 105 Hz in 1.0 M KOH. All polarization curves were corrected for the iR compensation in the three-electrode system according to the following equation: Ecorr = Emea − iRs (Ecorr: iRcorrected potential; Emea: experimentally measured potential; Rs: equivalent series resistance extracted from EIS). The working potentials vs Ag/AgCl were converted to an RHE scale according to ERHE = EAg/AgCl + 0.198 V + 0.059 pH. Calculations of Turnover Frequency (TOF) and Faradaic Efficiency. The active sites of the catalytic electrodes can be approximately estimated by CV measurements carried out in 1.0 M KOH electrolyte at a scan rate of 50 mV s−1 in the potential window from 1.0 to 1.6 V vs RHE for OER. The turnover frequency (s−1) can be estimated from

been made, it is still a great challenge to obtain such bifunctional electrocatalysts with low overpotential and longterm stability. In recent years, earth-abundant transition-metal dichalcogenides (TMDs), with the general formula MX2 (M = Fe, Co, or Ni and X = S or Se), are expected to serve as promising electrocatalysts to catalyze both HER and OER, such as CoS2,38 CoSe2,18,19 and NiSe2.39 According to the molecular orbital theory, TMDs are potential candidates as bifunctional electrocatalysts because of their proper surface with proton acceptor and hydride acceptor sites, which can promote the electron transfer between the surface cation and adsorbed reaction intermediates.40 Many strategies have been proposed to enhance the catalytic performance of these materials. However, the low electrical conductivity of most TMDs causes limited charge transfer and passivated electrocatalytic activity. Different from semiconducting pyrites such as FeS2 or NiS2, NiSe2 is a Pauli paramagnetic metal with a resistivity below 10−3 Ω cm,41 which makes it advantageous as an electrocatalyst. Despite exploration of its use as electrocatalysts for decades,42 only a few scattered studies have focused on it to date. The electrocatalytic activity is less satisfactory probably due to the poor electrical contact between catalysts and support, and the catalysts are easy to peel off during water splitting.12 In order to solve this problem, people endeavor to grow electrocatalysts on current collectors directly. The most common methods are electrodeposition or solvothermal routes on various substrates, for example, NiSe2 on a Ti plate by the electrodeposition method39 and CoSe2 nanoparticles on carbon fiber paper (CFP) by thermal pyrolysis.18 However, further applications are constrained by the complex fabrication process and insufficient electrode stability. It is important to develop a facile method to fabricate cost-effective NiSe2 catalysts with high electrochemical activity and stability toward water splitting. Here, we report the synthesis of novel 3D porous NiSe2 nanowrinkles anchored on Ni foam by an immersion− selenization strategy. The obtained NiSe2/Ni catalyst allows direct use in the device and avoids the use of a binder/ conducting agent, which demonstrates efficient and robust electrochemical activity toward both HER and OER. The asprepared electrode achieves the lowest overpotentials of 166 mV for HER and 235 mV for OER at 10 mA cm−2. The electrode was also applied as an integrated non-noble highperformance electrocatalyst for overall water splitting in an alkaline solution, achieving 10 mA cm−2 at a cell voltage of 1.64 V. Furthermore, the catalyst exhibits sustainability up to 80 h at 10 mA cm−2 with negligible increase in the potential, showing excellent long-term durability toward overall water splitting.

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EXPERIMENTAL SECTION

Materials Synthesis. Synthesis of Nickel Oxalate Nanosheets Grown on Ni Foam. In a typical run, the commercial Ni foam was degreased by sonicating in acetone for 5 min and rinsed with 1 M HCl and deionized water to remove NiO and residual organic species on the surface thoroughly. Then the cleaned Ni foam with an area of 2 × 4 cm2 was immersed in a 0.5 M oxalic acid ethanol solution with 5 wt % water at a constant temperature of 45 ± 2 °C for 2 h. After immersion, nickel oxalate (NiC2O4·2H2O) formed on the skeleton surface of the Ni foam, and the as-obtained sample was designated as NiC2O4/Ni for simplicity. Afterward, the NiC2O4/Ni sample was rinsed with deionized water and anhydrous alcohol and dried in air. Synthesis of NiSe2 Anchored on Ni Foam. The as-synthesized selfsupported NiC2O4/Ni foam (2 × 2 cm2) was placed at the center of

TOF = j × Selectrode/(2F × n) where j is the current density (A cm−2) for the different electrodes during the LSV measurement in 1.0 M KOH, F is the Faraday constant (C mol−1), and n is the number of active sites (mol) for the different electrodes. To ensure that the oxidation current originates from oxygen evolution rather than other side reactions and to calculate the Faradaic 2232

DOI: 10.1021/acssuschemeng.7b03657 ACS Sustainable Chem. Eng. 2018, 6, 2231−2239

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ACS Sustainable Chemistry & Engineering efficiency of the system, rotating ring-disk electrode (RRDE) tests were conducted on an RRDE configuration (Pine Research Instrumentation, USA) consisting of a glassy carbon disk electrode and a Pt ring electrode. The ring potential was held constantly at 0.40 V vs RHE to reduce the O2 formed from the catalyst on the disk electrode in the N2-saturated 0.1 M KOH solution. A continuous OER (disk electorde) → ORR (ring electrode) process occurred on the RRDE. The Faradaic efficiency (ε) was calculated as follows:

ε = Ir /(IdN ) where Id denotes the disk current, Ir denotes the ring current, and N denotes the current collection efficiency of the RRDE, which was determined using the same configuration with an IrO2 thin-film electrode to be 0.2. To properly calculate the Faradaic efficiency of the system, the disk electrode was held at a relatively small constant current of 200 μA (∼1 mA cm−2). This current was sufficiently large to ensure an appreciable O2 production and sufficiently small to minimize local saturation and bubble formation at the disk electrode.44

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RESULTS AND DISCUSSION The 3D NiSe2/Ni electrode was obtained by facile chemical immersion and subsequent selenization, as illustrated in Scheme 1. During the first immersion in the oxalic solution, Scheme 1. Schematic Illustration Showing the Two-Step Fabrication Strategy of NiSe2 with a Porous Nanowrinkle Structure Directly Grown on the Skeletons of 3D Ni Foam

Figure 1. (a) XRD pattern, (b, c) SEM images, (d) TEM image, (e) HRTEM image, and (f) corresponding SAED pattern of NiSe2/Ni.

The high-resolution TEM (HRTEM) images (Figure 1e and Figure S4c, d) clearly reveal the lattice fringes with interplanar spacings of 0.27, 0.24, and 0.21 nm, corresponding to the (210), (211), and (220) planes of NiSe2, respectively. The SAED pattern (Figure 1f) displays several discrete rings, further verifying the nanocrystalline nature of the NiSe2 nanowrinkles. And the diffraction rings can be indexed to (210), (211), (220), (311), (321), (421), (332) planes of NiSe2. XPS as a surface-sensitive spectroscopic technique can confirm the elemental compositions of samples. The high resolution XPS survey spectrum of NiSe2/Ni shows the peaks of Ni and Se elements, with the signals of O and C owing to contamination/oxidation of the sample (Figure 2a).46 As demonstrated in Figure 2b, the peak at 853.7 eV for Ni 2p3/2 can be attributed to Niδ+ arising from the Ni foam substrate, which shifts positively by 1.1 eV compared with the metallic Ni peak at 852.6 eV. The peak at 855.5 eV is due to Ni2+ for the surface oxidation states of Ni in NiSe2.47−49 For Ni 2p1/2, the peaks corresponding to Niδ+ and Ni2+ in the Ni−Se compound locate at 871.1 and 873.2 eV, respectively. The peaks at 860.7 and 879.0 eV are ascribed to satellites of Ni 2p3/2 and Ni 2p1/2, respectively.47−50 In Figure 2c, the Se 3d peak located at 54.9 eV implies the presence of Se22− and the broad peak at 58.8 eV is assigned to Se−O bonding at the surface.51,52 The existence of SeO2 in the solution during CV measurements can be

NiC2O4·2H2O nanosheets grew on the skeleton surface of the Ni foam (Figure S1). The NiC2O4·2H2O nanosheets were further transformed into NiSe2 nanowrinkles during the subsequent selenization treatment. Optical photographs of Ni foam, NiC2O4/Ni, and NiSe2/Ni are shown in Figure S2. The XRD pattern of the selenized Ni foam presents diffraction peaks well indexed to the planes of cubic pyrite-type NiSe2 phase (PDF No. 65-1843) and cubic Ni phase (PDF No. 652865) (Figure 1a). SEM images (Figure 1b, c and Figure S3a− c) show that the entire surface of Ni foam is uniformly covered with porous NiSe2. The porous NiSe2 is composed of numerous nanowrinkles, the thickness of which ranges from several tens of nanometers to 100 nm. The unique nanowrinkle structure of NiSe2 originates from the inheritance and further self-assembly of NiC2O4·2H2O nanosheets,43 which is favorable for quick supply of electrolyte and short ion-diffusion distance.45 Through the cross-view SEM image (Figure S3d), the thickness of the NiSe2 layer is about 4.3 μm. The skeletal structure of the Ni foam was maintained completely after selenization, ensuring its direct use as an integrated 3D electrode for water splitting (Figure S2a). The TEM images (Figure 1d and Figure S4a, b) further confirm the nanowrinkle structure of NiSe2, which are consistent with the SEM results. 2233

DOI: 10.1021/acssuschemeng.7b03657 ACS Sustainable Chem. Eng. 2018, 6, 2231−2239

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Figure 2. XPS spectra of (a) survey scan, (b) Ni 2p, (c) Se 3d, and (d) O 1s of NiSe2/Ni.

log j + a, where η is the overpotential, a is the Tafel constant, b is the Tafel slope, and j is the current density) reflects the intrinsic property of catalysts as well as the reaction mechanism. The value of b is determined by the rate-limiting step of HER.34 The measured Tafel slope of Pt/C is 30.6 mV dec−1, which is consistent with the reported value.10 The Tafel slope of NiSe2/ Ni, Ni foam, and NiC2O4/Ni is about 92.3, 100.1, and 140.8 mV dec−1 respectively, revealing the superior catalytic activity of NiSe2/Ni over Ni foam and NiC2O4/Ni. Moreover, the Tafel slope of NiSe2/Ni falls within the range 40−120 mV dec−1, illustrating that the HER process of NiSe2/Ni is dominated by the Volmer−Heyrovsky mechanism.59,60 The exchange current density (j0) is another significant kinetic factor, which reflects the electrochemical reaction rate occurring on the electrocatalyst at equilibrium. The j0 can be obtained from the linear area of a Tafel plot, and the intercept at the zero overpotential (η = 0) corresponds to the j0 value (Figure 3c). The exchange current density of NiSe2/Ni for HER is 0.11 mA cm−2, which is an outstanding value compared with other TMDs-based catalysts (Table S3). To explore the HER kinetics, EIS investigations were conducted at an applied overpotential of 200 mV in 1.0 M KOH (Figure 3d). The equivalent circuit (Figure S6) is composed of a resistor (Rs) and two parallel combinations including a resistor (R1, charge-transfer resistance (Rct)) and a constant phase element (CPE1, CPE2). Rs and Rct are well correlated to the electrocatalytic kinetics. Rs represents the Ohmic resistance deriving from the electrolyte as well as all contacts, while Rct reflects the charge transfer resistance at the interface between the catalyst and the electrolyte. A common view is that a small Rct is favorable to fast charge transfer kinetics. The Nyquist plots present an obvious increase of the Rct from NiSe2/Ni to NiC2O4/Ni (3.7 Ω for NiSe2/Ni, 7.8 Ω for Ni foam and 21.6 Ω for NiC2O4/Ni). The trend of these Rct values is consistent with that of the Tafel slopes of the corresponding samples.

detected by the titration method. Determination of SeO2 by sodium hyposulfite titration in the medium of hydrochloric acid is established, which is based on the reduction of SeO2 and oxidation of KI.53 The positions and relative intensities of the Ni 2p and Se 3d peaks are consistent with the previous reports for NiSe2.54,55 Figure 2d presents the O 1s spectrum, which illustrates the existence of oxidized Ni species in the Ni−Se compound.56 The HER activity of NiSe2/Ni was first evaluated. Figure 3a shows the corresponding polarization curves with iR correction at a scan rate of 5 mV s−1. As expected, Pt/C exhibits the highest activity for HER with a near-zero overpotential. The NiSe2/Ni sample exhibits an onset overpotential of only 56 mV, which is slightly larger than that of Pt. At more negative potentials, the current density increases sharply and the H2 bubbles emerge vigorously from the electrode surface. The NiSe2/Ni electrode exhibits a much lower overpotential of 166 mV to deliver 10 mA cm−2 for HER than the reported catalysts (e.g., 243 mV for NiS2/CC,57 220 mV for Ni3S2/NF,58 210 mV for NiFe LDH/NF3). Table S1 compares the HER performance of NiSe2/Ni with other non-noble metal HER catalysts in alkaline solutions. The bare Ni foam and NiC2O4/Ni foam exhibit inferior HER performance compared with NiSe2/Ni, with extremely higher overpotentials of 285 and 310 mV respectively. Additionally, XRD analysis and polarization curves of NiSe2/Ni obtained at different sintering temperatures and times are shown in Figure S5. The crystal structure is always pyrite-type for the NiSe2 obtained under different sintering parameters (Figure S5a), but the HER performances of these NiSe2/Ni electrodes greatly differ (Figure S5b). The NiSe2/Ni obtained at 350 °C for 2 h exhibits superior catalytic performance compared to the others. The mass density (loading) of NiSe2 in these NiSe2/Ni electrodes is presented in Table S2. Figure 3b presents the Tafel plots derived from the fitted polarization curves. The constant b in the Tafel equation (η = b 2234

DOI: 10.1021/acssuschemeng.7b03657 ACS Sustainable Chem. Eng. 2018, 6, 2231−2239

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Figure 3. Electrocatalytic activities of NiSe2/Ni, NiC2O4/Ni, Ni foam and Pt/C electrodes in 1.0 M KOH for HER. (a) Polarization curves of NiSe2/ Ni, NiC2O4/Ni, Ni foam and Pt/C at a scan rate of 5 mV s−1. (b) Corresponding Tafel plots with associated linear fittings. (c) Exchange current density (j0) of NiSe2/Ni. The log |j| value is −0.96, and j0 was calculated to be 0.11 mA cm−2. (d) Nyquist plots of NiSe2/Ni, NiC2O4/Ni and Ni foam electrodes. (e) Long-term stability measurement of NiSe2/Ni at 10 mA cm−2 (inset: Polarization curves of NiSe2/Ni before and after the stability measurements). (f) The capacitive current densities at 0.775 V vs RHE as a function of scan rate for NiSe2/Ni, NiC2O4/Ni, and Ni foam.

overpotential, illustrating the facilitated reaction kinetics compared with the other two electrodes. The catalytic ability of NiSe2/Ni to OER was also assessed in O2-saturated 1.0 M KOH. Figure 4a shows the iR-corrected LSV curves of different samples at 5 mV s−1. All of the samples exhibit considerable OER performances, and meanwhile certain oxidation reactions of the NiSe2/Ni and NiC2O4/Ni are observed before the onset of OER. The oxidation peaks between 1.35 and 1.45 V vs RHE for the NiSe2/Ni and NiC2O4/Ni can be attributed to the transition from Ni2+ to Ni3+.62−64 Specifically, the NiC2O4/Ni electrode exhibits an overpotential of 310 mV at 10 mA cm−2. Meanwhile, the overpotential of bare Ni foam and IrO2 is 370 and 340 mV, respectively. In comparison, the overpotential at 10 mA cm−2 is only 290 mV for the NiSe2/Ni electrode (Figure S8). It should be noted that it is difficult to accurately determine the OER overpotential to achieve 10 mA cm−2 through the polarization curve due to the broad oxidation peak. Even at a higher current density of 100 mA cm−2, the NiSe2/Ni sample exhibits an overpotential of 337 mV. Table S4 presents a detailed comparison of OER performance for NiSe2/Ni with other

Apart from the electrochemical activity, durability is another critical factor to assess the performance of catalysts for practical applications. Galvanostatic measurements in N2-saturated 1.0 M KOH at 10 mA cm−2 were carried out to evaluate the stability of NiSe2/Ni, as shown in Figure 3e. The cathodic overpotential increased by only 12 mV (from 166 to 178 mV) after 38 h. In addition, the polarization curve of the NiSe2/Ni after galvanostatic measurements shows only a minute negative shift compared to the initial one, suggesting the excellent stability of NiSe2/Ni (inset of Figure 3e). It is reported that the electrochemically active surface area (EASA) has a proportional relationship to the number of active sites, which can be estimated by the electrochemical double-layer capacitance (Cdl).61 Figure S7 shows the CVs of NiSe2/Ni, Ni foam, and NiC2O4/Ni at different scan rates. The CVs show a typical rectangular feature of an electrical double-layer capacitor. The current density vs the potential scan rate has a linear relationship (Figure 3f), and its slope is the Cdl. The NiSe2/ Ni has the highest Cdl (35.7, 0.32, and 0.21 mF cm−2 for NiSe2/ Ni, NiC2O4/Ni, and Ni foam, respectively) and lowest HER 2235

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Figure 4. Electrocatalytic activities of NiSe2/Ni, NiC2O4/Ni, Ni foam, and IrO2 electrodes in 1.0 M KOH for OER. (a) Polarization curves at a scan rate of 5 mV s−1. (b) Tafel plots. (c) Nyquist plots of NiSe2/Ni, NiC2O4/Ni, and Ni foam electrodes at 1.5 V vs RHE. (d) Long-term stability measurement of NiSe2/Ni at 10 mA cm−2 (inset: Polarization curves of NiSe2/Ni before and after the stability measurements).

Figure 5. (a) LSV plots of NiSe2/Ni//NiSe2/Ni, NiC2O4/Ni//NiC2O4/Ni, and Ni foam//Ni foam water splitting systems in 1.0 M KOH at a scan rate of 5 mV s−1. (b) Long-term stability measurement of the NiSe2/Ni-based electrolyzer at 10 mA cm−2 (inset: a digital photograph showing the evolution of H2 and O2 gases from the electrodes).

enhancement of the OER activity after the durability test (inset of Figure 4d). Although the XRD pattern of the catalyst after OER indicates it is still the NiSe2 phase (Figure S10), the corresponding XPS analysis reveals the formation of NiOOH. The Ni 2p1/2 XPS spectrum of the NiSe2/Ni electrode after the OER tests (Figure S11) manifests a positive shift of 1.3 eV relative to that of NiSe2 before OER. The peaks at 872.4 and 874.5 eV are attributed to the Ni2+ and Ni3+, corresponding to the existence of Ni(OH)2 and NiOOH respectively,30 which proceeds as Ni(OH)2 + OH− → NiOOH + H2O + e− in alkaline electrolytes.64 It is safe to conclude that the nature of the real catalyst is the active NiOOH formed at the surface.65 Knowledge of the surface density of active site allows for direct comparison of intrinsic activities across different catalysts as expressed by their turnover frequencies (TOFs).66 An electrolytic potential of 1.57 V vs RHE is required to achieve a TOF of 0.1 s−1 for NiSe2/Ni. In contrast, electrolytic potentials of 1.61 and 1.63 V vs RHE are required, respectively, for NiC2O4/Ni and Ni foam to realize a

nonprecious metal OER electrocatalysts. Tafel slopes of all the electrodes were obtained from Tafel plots, as shown in Figure 4b. The Tafel slope of NiSe2/Ni (63.1 mV dec−1) is lower than that of NiC2O4/Ni (82.4 mV dec−1), Ni foam (71.6 mV dec−1), and IrO2 (80.2 mV dec−1), which confirms a higher OER activity of our NiSe2/Ni electrode. The Tafel slope of NiSe2/Ni is 31.7 mV dec−1 for the oxidation process or adsorption of oxygenated species on NiSe2 (Figure S9), indicating a fast oxidation or adsorption process. The EIS tests were conducted to investigate the kinetics of OER at 1.5 V vs RHE (Figure 4c). The calculated Rct of NiSe2/Ni (3.1 Ω) is significantly smaller than that of NiC2O4/Ni (32.8 Ω) and bare Ni foam (>200 Ω), which accounts for the higher OER activity of the NiSe2/Ni electrode. In addition, the long-term stability test shows a slight attenuation in the overpotential (from 235 mV to 259 mV) at 10 mA cm−2 after 72 h (Figure 4d), indicating the excellent OER stability of NiSe2/Ni. The value of 235 mV could be considered as the real overpotential for NiSe2/Ni to obtain the current of 10 mA cm−2. The LSV data demonstrate slight 2236

DOI: 10.1021/acssuschemeng.7b03657 ACS Sustainable Chem. Eng. 2018, 6, 2231−2239

Research Article

ACS Sustainable Chemistry & Engineering TOF of 0.1 s−1 (Figure S12b), suggesting a greater OER catalytic activity of NiSe2/Ni. In addition, the Faradaic efficiency was calculated to confirm that the observed current originates from water oxidation rather than other side reactions. A ring current of 0.041 mA and a disk current of 0.214 mA were detected (Figure S13), which verifies that the observed oxidation current catalyzed by NiSe2/Ni can be fully attributed to OER with a high Faradaic efficiency of 95.8%. Given that the NiSe2/Ni electrocatalyst exhibits excellent activity and stability for both HER and OER in strongly basic media, an electrolyzer was made in a two-electrode configuration using NiSe2/Ni as the anode and cathode for overall water splitting in 1.0 M KOH. For comparison, NiC2O4/Ni or Ni foam was also used to assemble electrolyzers (NiC2O4/Ni//NiC2O4/Ni or Ni foam//Ni foam). As shown in Figure 5a, NiSe2/Ni exhibits excellent performance for overall water splitting and requuires a cell voltage of only 1.64 V to reach 10 mA cm−2. The voltage is much smaller than those of NiC2O4/Ni//NiC2O4/Ni (1.82 V) and Ni foam//Ni foam (1.87 V). This cell voltage at 10 mA cm−2 is also superior to previously investigated catalysts, such as NiFe LDH/Ni foam (1.7 V),3 Co−P/Cu foil (≈ 1.65 V),67 Ni0.5Co0.5/NC (1.75 V),68 NiCo2S4 NA/CC (1.68 V),69 and other reported electrolyzers (1.8−2.0 V).6 The durability of the NiSe2/Ni// NiSe2/Ni electrolyzer was evaluated in 1.0 M KOH at 10 mA cm−2 for 80 h (Figure 5b). Under the electrolysis process, H2 and O2 bubbles were observed on the surface of the cathode and anode, respectively (inset in Figure 5b). Even at higher current density (30 mA cm−2, Figure S14), the cell voltage of the NiSe2/Ni//NiSe2/Ni electrolyzer displays negligible fluctuation during the whole process (55 h), indicating its excellent stability of overall water splitting. In addition, XRD was conducted to investigate the structural stability of NiSe2/Ni after the durability test (Figure S15). Obvious diffraction peaks can be detected and well indexed to NiSe2 and Ni. Moreover, the porous nanowrinkle structure is still preserved even after such a long durability test (Figure S16). The excellent catalytic performance of NiSe2/Ni toward overall water splitting could be relevant to the following factors. First, the electrical conductivity is improved significantly by self-supported growth of NiSe2 on Ni foam. Ni foam is highly conductive, and in situ growth of NiSe2 onto Ni foam is in favor of electrical contact between NiSe2 and the substrate. The NiSe2/Ni foam can be directly used as the electrode for electrocatalytic tests, free of any binder and conductive agent. Second, charge transfer is another essential factor to enhance the catalytic activity of catalysts. Based on the EIS results, the Rct value of NiSe2/Ni is the smallest among NiSe2/Ni, NiC2O4/ Ni and Ni foam toward HER and OER, illustrating that NiSe2/ Ni has faster charge transfer kinetics than NiC2O4/Ni and Ni foam. Besides, the uniformly distributed NiSe2 nanowrinkles with the porous structure are beneficial to the transportation of electrolyte and involved ions. Finally, the porous NiSe2 nanowrinkles afford a large EASA, providing numerous electroactive sites for HER and OER.

for OER. The exchange current density of NiSe2/Ni is as high as 0.11 mA cm−2 for HER. Moreover, the NiSe2/Ni-based electrolyzer requires a cell voltage of only 1.64 V to achieve the current density of 10 mA cm−2 for overall water splitting and maintains its activity for at least 80 h in 1 M KOH. The improved electrocatalytic activity and durability of the bifunctional NiSe2/Ni electrocatalyst can be attributed to the unique porous nanowrinkle structure, improved electrical conductivity, fast charge transfer kinetics, and large electrochemical surface area. This work will pave the way for development of selfsupported bifunctional TMDs-based electrocatalysts and create new opportunities for advancing clean energy systems.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03657. XRD pattern, SEM images, optical photographs, TEM images, LSV curves, equivalent circuit model, CV curves, Tafel plot, XPS spectrum, turnover frequency curves, ring and disk current, long-term stability measurement, and Tables S1−S4 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhonghua Zhang: 0000-0002-2883-4459 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by National Natural Science Foundation of China (51671115), and Young Tip-top Talent Support Project (the Organization Department of the Central Committee of the CPC).

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REFERENCES

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CONCLUSIONS In summary, the 3D porous NiSe2 nanowrinkles directly grown on commercial Ni foam can be fabricated by an immersion− selenization strategy. The NiSe2/Ni hybrid material can function as an active and robust bifunctional catalyst for both HER and OER in alkaline solutions. The overpotential to achieve 10 mA cm−2 is merely 166 mV for HER and 235 mV 2237

DOI: 10.1021/acssuschemeng.7b03657 ACS Sustainable Chem. Eng. 2018, 6, 2231−2239

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